improve doc for new contributors #2565

Job	Run time
build_system	24s
build (gcc-15.deb, --std-tests)	5m 56s
duplicates	2s
build (gcc-15.deb, --test-libcore)	14m 36s
build (gcc-15.deb, --extended-rand-tests)	8m 42s
build (gcc-15.deb, --extended-regex-example-tests)	6m 28s
build (gcc-15.deb, --extended-regex-tests)	12m 40s
build (gcc-15.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	11m 16s
build (gcc-15.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	11m 29s
build (gcc-15.deb, --projects)	11m 27s
build (gcc-15-without-int128.deb, --std-tests)	6m 13s
build (gcc-15-without-int128.deb, --test-libcore)	14m 43s
build (gcc-15-without-int128.deb, --extended-rand-tests)	8m 39s
build (gcc-15-without-int128.deb, --extended-regex-example-tests)	6m 42s
build (gcc-15-without-int128.deb, --extended-regex-tests)	12m 27s
build (gcc-15-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	11m 35s
build (gcc-15-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	12m 0s
build (gcc-15-without-int128.deb, --projects)	11m 10s
success	1s
	2h 46m 30s

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